Fix null YAML values being replaced by "null"

Related issues:

* https://github.com/kubernetes-sigs/kustomize/issues/5031
* https://github.com/kubernetes-sigs/kustomize/issues/5171

After noting this behaviour was not present in
d89b448c74 a `git bisect` pointed to the
change 1b7db20504. The issue with that
change is that upon seeing a `null` node it would replace it with a node
whose value was equivalent but without a `!!null` tag. This meant that
one application of a patch would have the desired approach: the result
would be `null` in the output, but on a second application of a similar
patch the field would be rendered as `"null"`.

To avoid this, define a new attribute on `RNode`s that is checked before
clearing any node we should keep. The added
`TestApplySmPatch_Idempotency` test verifies this behaviour.

See also https://github.com/kubernetes-sigs/kustomize/pull/5365 for an
alternative approach

kyaml/yaml/rnode.go

@@ -24,6 +24,20 @@ func MakeNullNode() *RNode {
 	return NewRNode(&Node{Tag: NodeTagNull})
 }
 
+// MakePersistentNullNode returns an RNode that should be persisted,
+// even when merging
+func MakePersistentNullNode(value string) *RNode {
+	n := NewRNode(
+		&Node{
+			Tag:   NodeTagNull,
+			Value: value,
+			Kind:  yaml.ScalarNode,
+		},
+	)
+	n.ShouldKeep = true
+	return n
+}
+
 // IsMissingOrNull is true if the RNode is nil or explicitly tagged null.
 // TODO: make this a method on RNode.
 func IsMissingOrNull(node *RNode) bool {
@@ -214,6 +228,9 @@ type RNode struct {
 	// object root: object root
 	value *yaml.Node
 
+	// Whether we should keep this node, even if otherwise we would clear it
+	ShouldKeep bool
+
 	Match []string
 }